1. Field of the Invention
This invention relates to new and improved curable adhesive compositions comprising amine and acid terminated polyamide resins, optionally together with epoxy resins, which are useful as curable hot-melt adhesives, potting compounds, molded products, sealants or coatings. These novel adhesive compositions are characterized by improved properties including high flexibility, low softening point and low tackiness, and good tensile strength, and are optionally used in the form of aqueous dispersions. The amine component of the polyamide resin comprises from about 90 to about 20 equivalent percent of 1,2-diaminopropane and from about 10 to about 80 equivalent percent of a piperazine-containing diamine. Optionally, up to about 20 equivalent percent of a C.sub.2 -C.sub.12 polyamine may be employed.
Polyamides which are flexible usually do not have high strength and low tack, since flexibility requires a low modulus of elasticity, which is usually consistent with tackiness. Surprisingly, it has been found that the polyamides of the present invention which contain 1,2-diaminopropane possess not only a low modulus of elasticity, but also have low tackiness. These polyamides have excellent open times of 0.5 to 5 minutes; long enough to allow good bonding, but short enough to allow assembly line product. Most open times are less than a minute. This combination of properties provides compositions which are good adhesives for lamination applications. The reduced tack allows the adhesive polyamide compositions to be used to make coated roll stock. High strength together with flexibility gives products which can be used in many industrial applications where these properties must coexist, e.g., in book binding and shoe lasting. An optional embodiment of the present invention involves the use of the novel polyamides described herein to cure epoxy resins;
2. Brief Description of the Prior Art
It is known that dimer-based polyamides are useful for curing epoxy resins, as in U.S. Pat. Nos. 2,999,826 (Peerman et al.), U.S. Pat. No. 2,930,773 (Renfrew et al.), and U.S. Pat. No. 2,881,194 (Peerman et al.). The liquid polyamides described therein are mixed with liquid epoxies and allowed to cure. The resulting products possess little cohesive strength until sufficient curing occurs. Certain of the cured products are useful as adhesives, but have shortcomings in that such products are usually very rigid and possess limited ductility and flexibility.
Curable hot-melt compositions have been made using polyamide resins and epoxy resins as broadly described in U.S. Pat. No. 2,705,223 (Renfrew et al.). The polyamides comprise the condensation products of polymeric fatty acids with aliphatic polyamines. Compositions varying from 10% epoxy resin and 90% polyamide resin to 90% epoxy resin and 10% polyamide resin are disclosed. Typically, the compositions taught by Renfrew do not possess good adhesive strength upon cure and provide limited working time after the mixing of the components. In addition, such compositions exhibit poor flexibility and poor resistance to heat, water and organic solvents when applied to substrates at ambient temperature.
The prior art teaches that adhesives having improved flexibility can be obtained by incorporating piperazine into polyamide resins. For example, U.S. Pat. No. 4,853,460 (Harman) discloses an uncured polyamide composition wherein the amine component consists of about 25 to 75 percent equivalents of a cyclic diamine such as piperazine, a non-cyclic aliphatic diamine in which the amine groups are bonded to odd-numbered carbon atoms on the aliphatic chain, such as 2-methyl-1,5-pentanediamine, and one or more alkylene diamines. The resulting compositions, however, exhibit more tackiness than the compositions of the present invention. Changes in the formulation to lower the modulus, lead also to a large decrease in strength. In addition, the low modulus and low viscosity materials suffer from cold flow, i.e., they creep at ambient temperatures. U.S. Pat. No. 4,082,708 (Mehta), teaches an adhesive system comprising an epoxy resin and a polyamide wherein the polyamide is derived substantially from 1,4-(bis-aminoalkyl)piperazine. Although useful as cured products, these piperazine-based polyamide compositions exhibit poor recovery and limited flexibility. In addition, the piperazine-containing polyamides readily absorb aqueous and organic solvents and hydrolyze easily. It has been proven to be particularly difficult to achieve hot-melt compositions having good solvent and water resistance using the piperazine-containing polyamides of the prior art.
In U.S. Pat. No. 4,566,931 (Panoch et al.), there is described a heat sealable non-cured adhesive consisting essentially of copolyamides produced from omega amino carboxylic acids or lactams having 6 to 12 carbon atoms, dicarboxylic acids, and alpha-omega diamines wherein at least about 30% by weight of the diamines consist of singly branched diamine having 6 carbon atoms.
The use of 1,2-diaminopropane in preparing polyamide resins is also known in the art, as for example in U.S. Pat. No. 3,408,317 (Vertnik); however, there it is used preferably in a minor proportion along with ethylene diamine, and 1,3-diaminopropane is taught to be essentially equivalent thereto. U.S. Pat. No. 3,377,303 (Peerman) recites 1,2-diaminopropane among other diamines which may be used together with an essential piperazine-containing diamine component of the polyamide. However, only ethylene diamine is demonstrated in actual use, and there is no appreciation of the unique properties conveyed by use of 1,2-diaminopropane.
While various diamines have been used to prepare adhesive polyamide resins heretofore, there has been no suggestion of the unexpected properties obtainable with the particular combination of 1,2-diaminopropane and piperazine-containing diamines which characterize the novel polyamide compositions of the present invention.